Systems and methods for capturing and interpreting audio

ABSTRACT

As part of a system, a device is provided as part of a system, the device being for capturing vibrations produced by an object such as a musical instrument. The device has a first sensor placed in contact with a surface of the drum, such as a rim of the drum, and a second sensor placed at a fixed location relative to the drum, but not touching the drum. A method may be provided for interpreting the output of the sensors within the system, the method comprising identifying the onset of an audio event in audio data, selecting a window in the data for analysis, applying transforms to generate a representation of the audio event, and comparing that representation to expected representations in a model.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Patent Application No. 62/193,233 filed Jul. 16, 2015, Provisional Patent Application No. 62/055,037, filed Sep. 25, 2014, and Provisional Patent Application No. 62/055,024, filed Sep. 25, 2014, each of which are incorporated by reference herein.

FIELD OF THE INVENTION

The invention relates to capturing and interpreting audio, and for using interpreted audio to control computer systems. Specifically, this disclosure relates to hardware and software components for systems for capturing and synthesizing percussion instruments.

BACKGROUND

Many traditional acoustic musical instruments, such as percussion instruments, cannot be easily emulated or synthesized by electronic systems. While attempts have been made to build electronic drums, such electronic drums do not currently reproduce the sound of acoustic drum kits, and the subtlety of an acoustic performance may be lost by using existing electronic equivalents of drums.

Modern electronic drum kits are typically activated using a set of binary triggers, such that striking an electronic drum pad at a trigger will produce a specific sound. However, an acoustic drum kit can produce a much wider variety of sounds by using the main drum pad as a continuum, rather than a series of discrete triggers, using the rim of the drum as part of the instrument, and by striking a drum with different materials or utilizing different techniques, each activating the acoustics of the physical object in different ways to produce different sounds. For example, drummers may make unique sounds by hitting the rim of a drum or a side of a drum, or other locations where electronic devices may not have triggers. While some electronic drum pads can distinguish between harder and softer hits, they are still limited to which trigger is activated and at what force.

Traditionally, acoustic drum sounds have been captured by standard acoustic microphones that are prone to also detecting ambient sounds other than those emanating from the drums. Such ambient sounds may include unwanted sounds that are difficult to isolate during processing. Further, such microphones may create signals that are usable to recreate the specific audio from the performance captured, but which cannot be used to modify or refine playback of the performance, since such signals are difficult or impossible for a computerized system to interpret. Further, such signals cannot be easily used to control a computer and cause customized playback of audio other than an amplified version of that captured.

Further, existing electronic drum kits require musicians to familiarize themselves with a new set of equipment that looks and feels different from what they are used to. Drummers are typically comfortable with their kit, and they are proficient at executing special drumming techniques on the equipment they have used for years.

There is a need for a system that can emulate and synthesize percussion instruments without losing the benefits of the acoustic and analog nature of the original instrument. There is a further need for such a system that can interpret signals captured from such percussion instruments and utilize them to control the output of a computer system. There is a further need for such a system that is adaptable to equipment that percussionists use currently and are comfortable with without the limitations of traditional microphones.

SUMMARY

The present disclosure is directed to systems and methods for capturing and interpreting audio, as well as outputting a sounds selected based on the interpretation by the systems and methods. Also disclosed is a device for use in conjunction with the system as well as methods for training such a system.

A device is provided as part of a system, the device being for capturing vibrations produced by an object such as a musical instrument. The device comprises a fixation element, such as a clamping mechanism, for fixing the device to the musical instrument. In the description that follows, the musical instrument is a drum, but devices may be provided for other instruments as well. The device has a first sensor placed in contact with a surface of the drum, such as a rim of the drum. This may be by placing the first sensor within the fixation element. The device also has a second sensor placed at a fixed location relative to the drum, but not touching the drum. The second sensor may, for example, be suspended above the head of the drum and attached to the fixation element or a housing associated with the device.

In this device, the first sensor may be a sensor for directly detecting vibrations in the rim of the drum, such as a piezoelectric sensor, while the second sensor may be for detecting vibration in the drum head without touching it, such as a coil sensor paired with a magnet placed on the drum head or a high speed camera or laser based sensor. The second sensor may be mounted with adjustments for moving the sensor with respect to the drum head.

The device may have a tapered or curved striking surface mounted above the second sensor to protect the sensor and to act as an additional surface for a drummer to hit while playing the drum on which the device is fixed.

The clamp acting as a fixation element may have a lower arm that has differently shaped sections for gripping different types of drums, and the device itself may be provided with on board circuitry and potentiometers for mixing signals from the different sensors.

The device may be used within a system, and may output signals from the two sensors, or a single mixed signal from the two sensors, to an audio interface, and the resulting signal may be used by a method for producing audio from electric signals within a data processing device.

In some embodiments, the system does not require a housing and comprises a first sensor in contact with a surface of the object, a second sensor spaced apart from the first sensor and located relative to the object, but not touching the object, and a processor for identifying an audio event based on signals captured at the first sensor and the second sensor. The identification may be performed by a method designed to interpret the output of the sensors.

Accordingly, a method may be provided for receiving a stream of audio data, identifying an onset of an audio event in the data, and selecting a discrete analysis window from the audio data based on the location of the onset of the audio event.

Using the contents of the analysis window, the method may then proceed to identify the audio event in the audio data by generating an n-dimensional representation, or other such signature, of the audio event and comparing the representation to expected representations of audio events along a plurality of the n dimensions.

Once the audio event is identified, the method may output a sound selected based on the classification of the audio event.

The n dimensions used to represent the audio event may each represent different aural qualities of the audio event, such as tonal or timbre components.

The audio data may be a time domain audio signal and the analysis may include transforming the signal to the frequency domain using a Constant-Q transform or a Mel-Frequency Transform. Once the n-dimensional representation is generated, it may be compared geometrically with a plurality of audio zones defined by expected signal parameters in at least two of the n dimensions associated with a sample sound.

Such a geometric comparison is used to map audio events to geometric zones associated with sample audio events, and when a corresponding zone is selected, a corresponding sample sound may be output.

In some embodiments, where the n-dimensional representation does not match any one zone, the geometric interpretation may be used to weight the sound across multiple zones. In some embodiments, this may be done separately across different dimensions, and sample sounds associated with different zones may be blended to achieve a blended output. In some embodiment, each dimension may be provided with a confidence level to determine how likely it is to be associated with a selected zone.

In some embodiments, a training method is provided for training such a system, the method comprising selecting an audio event to implement into the model and generating the audio event. Typically, a user will do this to train multiple audio events. In some embodiments, a user may also train the method to ignore certain audio events, such as a kick of a bass drum during a drum performance. In such an embodiment, the user will indicate to the system that an audio event should be ignored and then perform the audio event.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an implementation of a system for capturing and synthesizing audio from musical instruments.

FIGS. 2 and 3 are left and right side views of a device for capturing sounds from a percussion instrument.

FIGS. 4 is a left side perspective view of the device of FIG. 1.

FIG. 5 is a partially sectioned left side perspective view of the device of FIG. 1.

FIG. 6 is a bottom perspective view of the device of FIG. 1.

FIG. 7 is a right side perspective view of the device of FIG. 1 with a sensor extended therefrom.

FIG. 8 is a left side perspective view of a second embodiment of a device for capturing and synthesizing audio from musical instruments.

FIG. 9 is a perspective view of the device of FIG. 1 mounted on a drum.

FIG. 10 is a circuit diagram illustrating onboard mixing circuitry for the device of FIG. 1.

FIG. 11 shows a schematic for a signal flow for implementing the method of producing sound from electronic signals.

FIG. 12 is a flowchart for a method of onset detection within the schematic of FIG. 11.

FIG. 13 is a flowchart for a method of spectral analysis within the schematic of FIG. 11.

FIG. 14 is a flowchart for classification of audio signals within the schematic of FIG. 11.

FIGS. 15-17 illustrate the classification of audio signals within a system utilizing the schematic of FIG. 11.

FIGS. 18-19 illustrate an exemplary graphical user interface for implementing the systems and methods described.

FIG. 20 is a schematic diagram illustrating the device of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The description of illustrative embodiments according to principles of the present invention is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description. In the description of embodiments of the invention disclosed herein, any reference to direction or orientation is merely intended for convenience of description and is not intended in any way to limit the scope of the present invention. Relative terms such as “lower,” “upper,” “horizontal,” “vertical,” “above,” “below,” “up,” “down,” “top” and “bottom” as well as derivative thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) should be construed to refer to the orientation as then described or as shown in the drawing under discussion. These relative terms are for convenience of description only and do not require that the apparatus be constructed or operated in a particular orientation unless explicitly indicated as such. Terms such as “attached,” “affixed,” “connected,” “coupled,” “interconnected,” and similar refer to a relationship wherein structures are secured or attached to one another either directly or indirectly through intervening structures, as well as both movable or rigid attachments or relationships, unless expressly described otherwise. Moreover, the features and benefits of the invention are illustrated by reference to the exemplified embodiments. Accordingly, the invention expressly should not be limited to such exemplary embodiments illustrating some possible non-limiting combination of features that may exist alone or in other combinations of features; the scope of the invention being defined by the claims appended hereto.

This disclosure describes the best mode or modes of practicing the invention as presently contemplated. This description is not intended to be understood in a limiting sense, but provides an example of the invention presented solely for illustrative purposes by reference to the accompanying drawings to advise one of ordinary skill in the art of the advantages and construction of the invention. In the various views of the drawings, like reference characters designate like or similar parts.

A hardware system is combined with software methods to capture sounds from a musical instrument, interpret those sounds, and use the generated signals to control a computer, such as controlling the audio output of a computer. Such a system may emulate or synthesize the sound captured, or it may instead output audio samples mapped to those produced by the musical instrument. Mapped audio samples may be new sounds not sonically related to the actual sounds of the musical instrument, but rather audio structurally related to the acoustics of the instrument and the musicians way of interacting with it.

The hardware components described include a device comprising multiple sensors that can be used to capture sound from a musical instrument, referred to herein as both a device and a microphone. The captured sound is converted to an electrical signal which may be processed at a computer system using a variety of software methods. Similarly, the software methods disclosed herein may be utilized to interpret signals extracted from hardware components other than those described to identify and emulate or synthesize audio for a musical instrument. It will further be understood that while the embodiment disclosed relates to percussion instruments, specifically drums, similar hardware and software may be employed to capture and emulate sounds from other musical instruments and acoustic objects as well.

FIG. 1 shows an implementation of a system for capturing and synthesizing audio from drums. As shown, the system comprises several devices 100 for capturing audio from drums. Identical devices 100 can capture audio from a variety of drum types, including snare 110, tom 120, or kick 130 drums.

The audio captured is transmitted as an analog signal to a pre-amp or audio interface with analog-to-digital conversion 140 which processes the audio signals and then further processes the audio, selects an audio sample to output, and then generates an output signal to transmit to an audio output, such as a PA system 145 or a headphone monitor 147. In some embodiments, the audio interface transmits a resulting digital signal to an external computer 150 for further processing and for selecting an audio sample or applying an audio synthesis process and generating an output signal. In such embodiments, the computer 150 may be connected to an audio amplifier or speakers for outputting audio signals in real time, or it may be configured to store the results of the analysis or a recording of an audio output. In some embodiments, the computer 150 or the audio interface 140 may be connected to other hardware devices, such as lighting systems or hardware synthesizers, that may be controlled by the system via an interface to allow for user designed output profiles. For example, control messages may be output as generic MIDI messages that can be routed outside the system.

This system may be used for a real time performance, in which case audio is captured from each drum 110, 120, 130 of a drum kit using the devices 100, transmitted to the audio interface 140 for processing, either processed by an onboard processor or sent to the computer 150 for further analysis and classification, and transmitted to an amplifier for immediate playback of emulated or synthesized sounds. While the immediate playback may be of samples designed to sound as similar as possible to the acoustic playback of the drum kit, it may also be playback of alternative samples or synthesized sounds designed to give the drum kit a different sound profile, such as that of a different drum kit, a different type of drum, or distinct samples unrelated to traditional percussion performance. Further, the signal may be interpreted and used as a control signal for functions other than audio, such as hardware synthesizers, lighting, or other devices.

FIGS. 2 and 3 are left and right side views of the device 100 for capturing sounds from a percussion instrument, FIG. 4 is a left side perspective view of the device 100, FIG. 5 is a partially sectioned view, FIG. 6 is a bottom perspective view, and FIG. 7 is a right side perspective view of the device 100 with a sensor extending therefrom. FIG. 8 is a left side perspective view of a second embodiment of the device 100, and FIG. 9 is a view of the device 100 mounted on a drum 110.

The device 100 has a fixation element 200 for fixing the device to the musical instrument, a first sensor 210 to be fixed in contact with a rim 220 of the drum 110 to transduce vibrations from the rim or shell, or other rigid elements, of the drum, and a second sensor 230 suspended by the fixation element at a fixed location relative to the drum. The second sensor 230 transduces vibrations from the drum head 225, or other soft membrane elements. It will be understood that in the case of musical instruments other than drums, the second sensor 230 may be otherwise suspended at a fixed location relative to the musical instrument. All of these components may be installed within a housing 240 designed to arrange the described components with respect to each other and retain the sensors 210, 230, at specific locations with respect to the drum 110.

The fixation element 200 may be a clamping mechanism, and the first sensor 210 may be mounted within a grip 250 of the fixation element 200 so that it is secured to the rim 220 of the drum 110. The first sensor 210 may be of any type that can extract a signal from vibration of the rim 220, such as a piezoelectric element. When the fixation element 200 is secured to the drum rim 220, the first sensor 210 may then detect and capture vibrations on the rim and shell of the drum.

Alternatively, the first sensor may be a piezoelectric filament embedded inside the housing 240 of the device, rather than within the grip 250, and placed over the rim of the drum adjacent the upper clamp, or grip 250, of the fixation element. A small (approximately 20 mm) ceramic piezo disc element or a Polyvinylidene fluoride (PVDF) piezo film of similar size may be used. This transducer, or sensor, 210 may then pick up vibrations from the rim and shell of the drum. While piezoelectric sensors are described, other types of sensors are contemplated as well.

The second sensor 230 is suspended from the housing 240, or from the fixation element 200 or an extension of the fixation element, and is selected to detect vibrations in the drum 110, specifically the drum head 225, and exclude ambient sound. For example, the second sensor 230 may be a coil sensor, such as an electromagnetic coil pickup, for detecting a vibration in the drumhead 225. Such an electromagnetic coil pickup may be made by wrapping copper thread around an iron core, and a suitable coil may be a small telephone pickup coil, such as those typically used to record phone conversations directly from telephone receivers.

Other types of sensors are contemplated for the second sensor 230 as well, such as high speed cameras or laser based sensors. When using a high speed optical camera in place of the coil or laser, the camera is aimed at the membrane of the drumhead, which transmits video of movements of the membrane. An audio signal is deduced by analyzing changes in the video feed, such as, for example, a circular marker on the membrane whose diameter will appear larger or smaller to the camera based on its proximity. The fluctuations in diameter act as a measurement of the membrane's vibrations.

Where the second sensor 230 is an electromagnetic coil sensor, it may be paired with a small magnet 235, such as a rare-earth magnet, fixed to a surface of the drum head. The magnet may be fixed to the surface of the drum using glue or transfer adhesive, and it may be provided in the form of a disposable permanent or one-time use sticker.

As shown in FIG. 7, the second sensor 230 may extend from the housing 240 of the device. The second sensor 230 may be provided with adjustments 275 for moving the sensor parallel to or perpendicular to the drum head for positioning the sensor, or adjustments may be provided to lower the sensor at an angle towards the drum head. Once the second sensor 230 is positioned, a cam lock 277 may be used to secure the location of the sensor. When the second sensor 230 is an electromagnetic coil sensor, the adjustments may be used to position the sensor directly above the magnet 235 fixed to the surface of the drum head and control the distance between the coil and the magnet, which controls the sensitivity of the transducer.

The housing 240 may further contain a thumb screw 260 for adjusting the fixation element 200 to fit a variety of musical instruments 110, 120, 130, as well as potentiometers 270 for adjusting the gain of each of the two sensors 210, 230. While two sensors, and two corresponding potentiometers, are shown and described, additional sensors or sensor types may be provided for increased data and accuracy.

Fixation element 200 may be a clamp, and may include hooks designed to fit a variety of instruments, such as various standard drum heads. As such, an upper portion of the clamp may be the grip 250 containing the first sensor 210, while the lower portion of the clamp may be a hook with a first segment 280 for gripping one type of drum, such as a bass drum, and a second segment 290 for gripping a second type of drum, such as a snare drum.

As shown in the alternative embodiment of FIG. 9, the device may further include a strike bar 300 which would both protect the internal components of the device from stray impacts, and may act as an independent striking surface for drummers to utilize to create unique sounds or effects. In some embodiments, the strike bar may be used to trigger visual effects, such as lighting sequences. The strike bar is mounted above the second sensor 220 such that it rests opposite the second sensor from the drum head. The strike bar 300 is tapered and/or curved such that striking the bar at different points along its length exhibits a timbre or tonal shift so that a wide variety of sounds may be made by striking the bar at different points. The strike bar 300 may be fixed to the housing 240 near the fixation element, or elsewhere on the housing, and cantilevered over the second sensor 230 such that a portion of the strike bar is suspended to allow for a variety of sounds.

Output may be transmitted to the pre-amp through a standard audio output port 310, such as a female XLR output jack.

In some embodiments, the device 100 may incorporate onboard mixing hardware within housing 240 to mix the signals from the first sensor 210 with the signals from the second sensor 230 and the output through the XLR interface 310 may then be provided as a mono audio channel.

FIG. 10 shows one example of onboard circuitry that may be implemented in the device of FIG. 1. The onboard circuitry may be comprised of active preamp and mixing circuits powered via phantom power at 48 volts carried along the output cable, or by an external power source with either a battery or power provided from a wall plug. These circuits actively condition the signals from each transducer and mix the signals for output. This is achieved with operational amplifier circuits, using a charge amplifier configuration for the piezoelectric signal conditioning, an inverting preamplifier configuration for the magnetic coil and two buffer op-amp circuits to mix the signals. Each signal has a variable potentiometer 270 with which the user can control the gain independently before mixing.

After mixing, the device 100 outputs a mono signal along a balanced line via the XLR port 310, which in turn connects to the pre-amplification unit and analog-to-digital converter 140 to supply the software system with a digital audio stream.

In some embodiments, the first sensor 210 and the second sensor 230 are provided without fixation element 200. In such cases, the first sensor 210 may be placed directly on the surface of the drum 110 and the second sensor may be suspended above the drum head 225. In some embodiments, a system may be provided for analyzing multiple drums 110, 120, 130 of a drum kit, and each drum may be provided with a first sensor 210, while a second sensor may be provided to capture vibrations from all drums of the drum kit.

The software methods described below may utilize the output of the device 100 discussed above with respect to FIGS. 2-10, but may, in the alternative, be applied to any physical object whose vibrations can be captured by a sensor or set of sensors. Acoustic musical instruments are ideal examples of these types of objects, and the methods are described with respect to analyzing drums.

Initially, the acoustic output is captured and translated into the digital domain through an analog-to-digital converter, such as audio interface 140, and is received by a computer 150 or a processor within the audio interface 140. Many aspects of the following description, including and not limited to the method of performing onset detection, data reduction, model design and selection, classification, design of graphical user interface and method of extracting control values from user generated events may have numerous modifications that can be made without departing from the spirit of the invention. Accordingly, specific values and thresholds are described, but are provided merely as examples, and may be replaced by other values as appropriate.

FIG. 11 shows a schematic for a signal flow for implementing the method of producing sound from electronic signals. A computer system 150 configured to operate the method first receives a stream of data, the stream of data comprising the audio data from the pre-amplifier 140 (1000). The method then identifies, in the audio data, the onset of an audio event (1100). The onset may be, for example, an impact of a drumstick with a drum head, and the resulting audio event may be the resulting reverberation of the drum head. The determination of the onset is described in more detail below with respect to FIG. 12.

Once an onset is identified in the audio data, a discrete analysis window is extracted from the audio data based on the location of the onset of the audio event. The analysis window is selected to capture enough time after the onset of the audio event to sufficiently identify the audio event.

After the discrete analysis window is extracted, the spectrum of the extracted segment of the signal is analyzed (1200) and is used to generate an n-dimensional representation of the audio event captured. These dimensions may be selected to capture aural components of the audio event specific to the instrument being captured, and such dimensions may be selected using a neural network trained on pre-labeled data. For example, in the case of drums, or other percussion instruments, these dimensions may be selected to capture the timbre of the audio event. This is discussed in more detail below with respect to FIG. 13.

The audio event is then classified (1300) by comparing the n-dimensional representation of the audio event to expected representations of audio events along at least some of those n-dimensions. This may be performed by using a user-trained model to compare previously identified audio events performed with the same equipment to newly captured audio events. This may also be performed by using a fully trained model learned from previously collected data. Such a model may not be trained by the user or on the particular equipment being captured, but may have a generalized knowledge of drum-related events and can perform classification based on that knowledge. In some embodiments, a fully trained model may be provided with the software as a model designed to map to a particular drum kit. Accordingly, such a model may be pre-trained for a large number of commonly used standard drum kits.

In some embodiments, the classification is then passed through a geometric interpretation engine (1400) to resolve audio events with respect to expected representations of audio events, such as those stored in a database or generated by a user trained model. This process is discussed in more detail below with respect to FIG. 14, and implementations of the process are illustrated in FIG. 15-17.

Finally, audio samples or audio synthesizing processes are selected to be played back based on the classification (at 1300) and geometric interpretation (at 1400), as well as user-programmed sound output mappings and are output as sound (1500). This sound may be saved as a new audio file or may be immediately output to an amplifier or speaker, as in the case of a live performance. The audio sample may be one stored in a database, or it may be created based on multiple stored samples combined based on the geometric interpretation (at 1400). In some embodiments, the audio sample selected for playback is generated entirely based on an algorithm drawing from the classification or geometric interpretation. While the process is described in terms of audio output, the output may be a control signal output for activating a non-audible event, such as a lighting sequence.

FIG. 12 is a flowchart for a method of onset detection 1100 within the schematic of FIG. 11. Initially, the computer system 150 receives the audio data from the pre-amplifier 140 (at 1000) and applies an initial spectral analysis to determine the nature of the onset of the event. This is to determine if the event occurring is a single impact, such as the striking of drum head 225 with a drum stick, or if it is a continuous event, such as a quick sequence of impacts (roll) or a more subtle audio event, such as an impact or sequence of impacts with a brush.

As audio comes into the system as a digital stream, it may be windowed by taking a set of samples and applying a Hann window function, or any other windowing function, to reduce spectral bleed during spectral analysis. The stream may be saved in a buffer to allow for overlapping analysis windows. In the embodiment shown, the computer system 150 initially applies a Fast Fourier Transform (FFT) to the window being analyzed (1110) in order to transform the block of audio being evaluated into a frequency spectrum representation.

Then, a multiband pooling transform may be applied to compress the frequency representation. For example, a Mel-Frequency Transform, or a Constant Q Transform can be applied here (1120). An onset function is then applied to weight and sum various spectral components (1130) to either select a peak to represent the onset (1140) or determine if a roll, or other continuous event, is occurring (1150).

The Onset function (1130) may include first whitening the signal, i.e., reducing resonance in the signal, by dividing by the signal's average spectral envelope. This acts to reduce false onsets when used with very resonant drums. Each band of the pooled representation may then be weighted to favor high frequencies, and the onset function may then convert the window to decibels, subtract from a previous window to create a differential between windows, applying half-wave rectifying algorithms, and then summing the windows. In the described onset function, onsets would be represented by high peaks in the function. While a single onset function 1130 is described in detail, other functions may be used to identify onsets of events as well.

The onset function (1130) is then fed into a peak picking algorithm that searches for peaks in the function while minimizing noise by using a mean adaptive threshold window and controlling for minimum onset distance (which may be, for example, between 2 and 3 milliseconds) and minimum threshold for onset within the context of the onset function. In one embodiment, the algorithm tests for the following three criteria, with sample time values provided for reference, to determine if an onset has occurred:

-   -   a. Is the current frame value larger than all previous frames         within a given window of time (about 3 milliseconds)?     -   b. Is the current frame value greater than a given threshold         value plus the average of previous values within a given window         of time (about 3-16 milliseconds)?     -   c. Has a previous onset occurred within a given window of time         (˜3 milliseconds)?

While the algorithm described has been tuned for use with drums, it may be adapted to work with other instruments by using corresponding onset functions.

The output of the onset function (1130) and/or the original audio frame is then also sent to an algorithm to detect sustained events, which are indicated by sustained or frequent peaks in the onset function. This algorithm may be used to detect specific musical gestures and techniques, such as rolls on a drum where a smooth sound is produced by pressing the sticks against the drum to create rapid bounces of the sticks on the head, or the presence of a continuous gesture such as that created by rubbing brushes (bundles of fine metal wire) on the head of the drum as is common in jazz music.

Some embodiments, in the case of a continuous gesture, may bypass the onset detection step (at 1100) and proceed to the timbre or spectral analysis stage (at 1200). This allows for analysis and processing of continuous sounds and timbral changes that are not preceded by detectable onsets.

In such a scenario, a neural network, such as that described below, may be trained to subsume the task of onset detection as well as classification and spatial projection of continuous events. Alternatively, the onset detection module may be utilized to recognize a pattern of continuous sound and trigger the analysis stage to perform continuous analysis until the onset detection module detects that the continuous event has ceased.

Once an onset has been detected within the length of the analysis window, a sample-accurate onset detection algorithm is applied in order to localize the beginning of the onset. A length of audio that contains the detected onset is bandpass-filtered to remove low frequency and high frequency components, emphasizing frequencies around 10 kHz. The frame is then halfwave rectified and processed to create a running maximum vector. This is calculated by examining each sample within the window in the direction of time, searching for new maximum values and saving the current maximum value to a vector. The start of the onset is determined by finding the index of the first value in this vector of running maximums that exceeds some threshold value. The vector of running maximums may be normalized so that its values lie in the range of 0 to 1 which allows for the use of a standard threshold value regardless of the original amplitude of the onset. This sample start value is then used to extract an audio frame that contains the beginning of the event or onset and extends into the onset for a given number of samples, referred to as an analysis frame or analysis window.

The method may require a fixed length of audio information for any given event in order to generate an analysis frame. The fixed length may be user selected, and the length of the frame results in an inverse relationship between latency between event and execution and accuracy of the system.

FIG. 13 is a flowchart for a method of spectral analysis 1200 within the schematic of FIG. 11. This spectral analysis 1200 may be a timbre analysis in the context of a percussion instrument, such as a drum. After an onset is detected (at 1100), and an analysis window is extracted, a Fast Fourier Transform (FFT) is applied (1210) to samples within the analysis window, followed by a Constant Q-frequency transform (1220) to the data series generated by the FFT. While the flowchart illustrates the application of an FFT followed by a Constant Q frequency transform, a variety of sequences may be applied in the analysis. For example:

-   -   a. The windowed data may be left as is, remaining in the         time-amplitude representation.     -   b. It may be projected into a log-spaced frequency         representation through the use of a Constant-Q transform.     -   c. It may be processed as in (b), and then projected to the         “quefrency” domain by applying a Discrete Cosine Transform. This         acts as a data compression step that preserves the frequency         structures present in the signal.     -   d. It may be processed as in (b) or (c), however using an         alternate frequency representation such as a mel-spaced (as with         Mel-Frequency Cepstrum Coefficient transforms) frequency         representation or a linearly spaced frequency representation (as         with a Discrete Fourier Transform).     -   e. It may be processed as in (b), (c), or (d) and then         dynamically compressed by taking the nth root of all its values         or by applying a decibel calculation.

Some of these combinations of transforms may be used to generate, for example, amplitude/phase against frequency for various log-spaced frequency bins.

Further, the transformations described above may be followed by feeding the results into a series of matrix transformations and nonlinearities as is common to neural network systems (1230). A neural network (shallow or deep), that may consist of a series of functions as matrix multiplications with weight and bias values (and subsequent nonlinear transformations, trained on previously collected data through either supervised/semi-supervised/or fully unsupervised methods as is common in machine learning tactics, may be applied to the output of the methods described above. This network of transformations may also be a convolution neural network, where trained weight kernels are multiplied convolutionally across the input.

Such a network may serve several purposes: it may be a fully trained model that results in discrete classification based on a posterior probability by reading the results of a soft-max operation on the network's output as probabilities for inclusion in a given class. It may also serve to project the input data into a new data space wherein data points are ordered in a way that relates to a perception of aural timbre. In this space, similar sounds will produce outputs that are near each other and dissimilar sounds will produce outputs that are farther from each other. This network may also represent a model that has been pre-trained to recognize the various timbres produced by striking a drum with either of the two methods described above, but that subsequently undergoes a calibration process during a user run calibration of the system, where the structure of the model or data space is transferred or transformed to match up to the timbral structure of a drum that is to be used with the system.

In one embodiment, a neural network may be built and trained to produce an embedding of audio data into a lower dimensional space that preserves the structure and relevance of a drum's sonic qualities. One way to implement such an embodiment is by creating a large dataset of labeled data by labeling recordings of a variety of different drums being struck in various ways. A multilabel approach works here as well, such that a frame of audio that contains the audio produced when hitting the drum in the center of the drum head with a wooden tip stick could be separately labeled “centerhead” and ^(“)wood stick tip.”

Similarly, another data point, produced by striking the drum on the rim with the tip of the stick would have the labels “rim” and “wooden stick tip.” Any given data point can have multiple labels that describe what the data point relates to in terms of striking acoustic drums. With this data, a network can be trained to create a model that predicts the characteristics of unlabeled data that is projected into this space. One approach would be to use a DRLIM architecture where a Siamese neural network that has identical structure and mirrored weights is given two arbitrary inputs. If these inputs have similar label sets then a cost function is applied and back propagated through both of the Siamese neural networks to encourage the outputs to be similar. If it is given dissimilar inputs, the same is done however the cost function updates the network to ensure that the outputs of the two networks are dissimilar. In this way, a transformation is obtained that projects data points into a space that is geometrically ordered based on the tags in the dataset.

The output of this analysis or spatial projection step may produce relatively low dimensional data points, and a frame of audio of length 1024 samples (23 ms) may be reduced to an arbitrarily small number of dimensions. This data is then used to classify the audio event (at 1300).

FIG. 14 is a flowchart for classification 1400 of audio signals within the schematic of FIG. 11. The results of the spectral analysis 1300 are used to generate an n-dimensional vector or matrix (1410) representing various features of the audio event analyzed. For example, the process described above in (c), utilizing the “quefrency” domain, may produce a data point that has 20 dimensions. The process described with respect to neural networks, on the other hand, may project this audio down to 2 or 3 dimensions, which may be readily visualized for the user. Such a process may also produce a higher dimensional output that represents a lower dimensional embedding of that audio frame in order to preserves relevant sonic qualities of the audio event outside of the easily visualized representation. These data points can then be classified as representing specific types of drum strikes and interpreted to have geometric and spatial significance within a continuous data space.

The vector generated may be used to classify the audio event based on a nearest neighbor model (1420) using density of data points in the model, which could in turn be used to classify the audio event (1430) as a specific type of audio event. Alternatively, the vector may be used to map the audio event based on a user mapped geometric interpretation of the vector (1440), which could then be used to generate a control value (1450) for audio output. These methods may be combined in order to weight the use of the nearest neighbor model with a control value or to use a geometric interpretation in some cases, (i.e., determining the location of an impact on a drumhead), while using a nearest neighbor interpretation in other cases (i.e., determining if a strike is on a drumhead or a rim of a drum).

During playback, each strike of the drum is captured, analyzed, and classified using the classification models described. When using a nearest neighbor model (at 1420), an unlabeled data point is compared to every data point in the model using a distance metric, such as Euclidean distance across several dimensions. Once the closest point in the model is discovered, the new data point is labeled with the same label as that of the closest data point.

In the case of a neural network classification model, the output of the classification model may be in the form of an energy density vector, with each dimension being associated with a different label. As such, multiple labels may be applied with different energy densities, which may be interpreted as confidence values that the data point should be associated with that label. Accordingly, in the example described above, high values in a dimension representing “center of drum” and in a dimension representing “tip of stick” will indicate that the data point represents a strike in the center of the drum using the tip of a drum stick.

As shown in FIG. 15, a label for a specified cluster may be applied to a new data point so long as that data point satisfies a density requirement for that cluster. In a given dimension, a ramp 1500 may be drawn between potential data points, and when a data point 1510 satisfies a density requirement for cluster A, it may be assigned a label associated with cluster A. Similarly, when a data point 1520 satisfies a density requirement for cluster B, it may be assigned a label associated with Cluster B.

The classification may then trigger an assigned event, such as the playback of a given sample.

When using a geometric interpretation (at 1440), including when using in conjunction with the classification methods already described, a geometric interpretation of a new data point within the model is derived in order to apply continuous control values and blend settings. Such continuous control values and blend settings may be user selected. This takes the data points within the data model used, and checks for relative distance, using a distance metric, such as Euclidean distance, to extract a relationship between the data point and the other classes, or zones, in the model.

There are several ways of extracting these relationships to convert a point in space relative to other regions of the space as a percentage of a distance between those regions. For example, when used in conjunction with a nearest neighbor classifier model, a given data point may be classified one way, but relate to other classes in the model based on relative distance to those classes in the data space. The model can be considered as a data space, or zone, given a measure of distance. Each of the points belonging to a given class in the model occupy a given region, or zone, of this space. An area of high density in this space can be interpreted as the region of the space that is most representative of that class (i.e. sound of the drum). We can call this the class's center.

As shown in FIG. 16, geometric interpretation involves calculating relative distances of a new data point to its labeled class' center but also to other relevant classes' centers in the model. Geometric interpretation in this case involves finding (A) the distance from the new point to its parent class's center and then finding (B) the distance from the new point 1600 to the center of any of the other classes in the model. This can be normalized and used as a control value for any continuous computer parameter such as the playback volume of an audio sample. In the case of a multi-label classifier, the values in each dimension can be interpreted in a similar way and averaged across certain dimensions to recover a relative distance between classes. As shown in FIG. 17, new classes, or clusters of data points, may be introduced into a model, and in such cases, the relative distance to each zone may be evaluated to properly evaluate a new data point 1700 using a geometric interpretation. Such calculations must therefore be updated to account for new cluster C. This is discussed below in more detail under the heading “Sound Mapping.”

Calibration and Training

The calibration or training of the models described above can function in two ways: It can be used to create a data model of a given drum that can be directly used for classification and geometric interpretation, or it can be used as a calibration step where a pre-trained model or neural network can be transformed to better fit the acoustics of the current drum being used in the system as in transfer learning architectures.

Accordingly, a classification model created in this step can be a nearest neighbors model that uses every labeled data point that the user provides (with statistical outliers removed based on the average density measurement of each class member) to determine the structure of the model. Alternatively, the data generated can be used to create a statistical model, such as a Gaussian mixture model, that can provide a more efficient means of providing a classification of unknown data points. In this step, the user will, one by one, instruct the system on the various sounds that the drum being played can produce.

During training or calibration, the software suggests a list of various regions and drum stroke combos that produce unique sounds specific to drumming that the user should provide. This includes drumstick strikes at the center of the drum head, strikes at the edge of the drum head, strikes at the rim of the drum with the tip and shoulder of the stick, separately, as well as common drum strokes such as cross-stick (where the tip of the stick rests on the head of the drum as the body of the stick is brought down to strike the rim) and rim-shot (where the stick strikes both the head of the drum and the rim simultaneously).

Besides this prescribed list, a user may provide custom strokes, regions, or sounds to be used in the training. The workflow may proceed as follows: a user may switch the software into training mode by pressing the “T” button in the graphical user interface. Once in training mode, the user interface may provide various regions on the screen that represent this list of regions on the drum and the strokes that can be played. These look like buttons or pads with one for each region. When in training mode, these pads can be selected. To train a given region, the user will select the pad that corresponds to that region by clicking on it with a computer mouse and striking the drum in the way that corresponds to the pad (e.g. to calibrate the center of the drum head, a user may select the pad labeled “1 Center” and then strike the drum repeatedly in the center of its head at varying velocities). Each strike of the drum saves a data point to the system that can be associated with that region. Once a satisfactory number of regions have been trained, the user may switch out of training mode by pressing the “T” button once more.

If the system is undergoing training in order to create a nearest-neighbors classification model, the user may train an arbitrary number of the pads. However, it is important that the user provide enough data points per pad to create a satisfactory model. Further, if the system is undergoing a calibration process in order to fine tune a previously trained network, for instance, it will be necessary for the user to provide data for a fixed number of regions. These two methods may be used in conjunction.

During the calibration step, a user may train a “void” pad to recognize ambient noise that erroneously triggers an event in the onset detection module. In a standard drum-set setup, for example, the kick drum will typically vibrate the snare drum, and these vibrations may trigger a false onset. By training the void pad to recognize these types of false trigger events based on their sonic content, they can be appropriately silenced.

Accordingly, in training a “void” pad, a user may first select a first audio event, such as a center hit, to be recognized by and implemented into the model used and perform that audio event at the drum. The user may then select a second audio event, such as a rim-shot to be recognized by and implemented into the model used and perform that audio event at the drum. The user may then select a third audio event, such as a kick drum impact, to be ignored by the model used and perform that audio event at the drum.

Sound Mapping:

After training, through the use of the GUI the user can “map” samples, synthesizers, effects, or any generic controls (such as MIDI), to be associated with note onset events related to the regions in a classification model. Further, control parameters can be mapped to relate distances between regions or other parameters of drum performance such as velocity or speed. An example of a control parameter might be continuous control messages of the MIDI protocol, e.g. the full distance range from class A to class B with a point P classified as A will identify a value on that range, which can be used as a generic continuous control value. When the user plays the acoustic instrument, an event is detected, the event is analyzed and classified and will then trigger its associated sound or control output. Another example of the translation of this data space to sound output is an option to blend or morph between two sounds associated with different regions in the data space. If one plays the instrument in such a way that each new data point moves progressively between two classes in the data space (e.g. beginning by striking the center of the drum and then striking the drum while progressively moving outward toward the edge will produce data points that begin very close to the class associated with the center of the drum and move progressively closer the class associated with the edge of the drum), the digital sounds associated with these two regions can be smoothly and progressively blended and output as audio through a speaker to correspond to the way the instrument is being played.

Extracting a continuous control value that relates a new data point to the class centers in the model's data space can be incorporated into the models discussed above with respect to FIGS. 15-17. In such an embodiment, a user may request a control value that identifies two classes as start and end points for the range of the control, say class A and class B, such as those shown in FIG. 16. Given a new data point 1600, the distance between data point P (1600) and the center of cluster A and data point P (1600) and the center of cluster B is calculated. A value between 0 and 1 is then calculated as follows: Value=dist(A,P)/(dist(A,P)+dist(B,P)) where dist is an n-dimensional Euclidean distance measure. This value can then be used to control any continuous parameter within the system or sent out to control an external parameter. An example of this is to control a variable parameter like a parameter in a digital effect unit such as reverb. Further low-pass filtering can be applied to the movement of these values as the drum is struck so as to achieve the effect of a knob being twisted continuously.

This value can also be used to create fuzzy classifications, where a new data point may be closer to one class than another but these relative distances are retained for controlling the relative volumes of two audio samples during playback. This will have the effect of a continuous aural movement in the digital output that relates directly to the continuous timbre movement of striking the drum repeatedly while progressing from one region to another. Similarly, other continuous control parameters may be extracted from other features of the data. For example, the velocity of a drum hit may be extracted from the force of a hit or the volume of the result of the hit, and the speed of a drum roll may be extracted from the distance between successive hits.

Graphical User Interface (GUI):

As shown in FIGS. 18-19, the graphical user interface has a representation of different sonic regions of a drum, a mixer to control individual channel volume, a sample browser, an effect controller creation panel, an effect panel with digital audio effect including reverb and delay, a sample editor/and drum synth parameter window, an onset detection parameter interface and a panel for selecting blend options between pad sounds.

The representation of different sonic regions of a drum shows several pads, each of which correspond to either a region of an acoustic drum, a stroke that can be used to play an acoustic drum or a specific way of striking the drum with a drum stick. Pads include, Center, Edge, Rimshot Center, Rimshot Edge, Crossstick, RimTip, RimShoulder, as well as extra pads for customizability and pads for the strike bar that sits on top of the hardware microphone.

The pads are arranged as a pie slice of a drum with the center being at the tip of the slice and the rim pads being at the edges.

The pads are selectable with a mouse click for both training mode and playing mode. In training mode, you select the pad to be trained. In playing mode, selecting a pad brings up contextual panels that show samples and effects that have been applied to the pad. Samples, effects, and synth can be assigned to a given by dragging and dropping either from the sample library, the effects rack or the synth rack.

As the user hits the drum, the corresponding pad will light up, indicating to the user that the system is recognizing the strike and correctly labeling it. If a blend has been set between two pads, you will see both pads light up as the drum is struck near the regions associated with those pads, showing varying light intensity depending on the relative values of each pad in that blend calculation.

The effect controller creation panel allows the user to create an effect controller that can use a variable method of striking the drum as a source (i.e. distance from center to edge as calculated in the geometric interpretation engine, force of strike, speed of successive strikes) and can then be applied to control any variable setting within the software. This will allow the user to control an effect like reverb decay by hitting the drum in particular places or by hitting the drum with varying volume or by hitting the drum quickly or slowly. These drumming elements are translated to continuous control values that are updated upon each strike of the drum.

The onset detection parameter interface allows the user to view the Onset Function in real time and adjust the threshold parameter as well as the adaptive window parameter (W3) interactively. Strikes of the drum appear as peaks in a function plotted on the screen. This function scrolls to the left as time passes. Peaks that are identified as events are marked with a blue line. Threshold is set by moving a horizontal line up or down. Peaks that remain under the horizontal threshold line will be ignored while peaks that exceed the threshold line will be marked as onsets. The adaptive threshold window is a rectangular box that can be extended to the right. Moving the corner of the rectangle to the right increases the adaptive threshold time window.

FIG. 20 is a schematic diagram illustrating the device of FIG. 1. As shown, the device 100 may have a housing 240, and the housing may contain a first sensor 210, a second sensor 230, and an output 310. The first sensor 210 is typically fixed against a surface of an object being captured by the device 100 and the second sensor 230 is typically not in contact with the object being captured, but rather is fixed in some position relative to the object. Vibrations captured by the first sensor 210 and the second sensor 230 may be combined by on board circuitry and then output at 310, or they may be output directly and then mixed externally to the device.

While the sensors are shown within a housing 240, the sensors may also be applied directly to surfaces, or may be suspended above drum heads or other instruments using separate stands. For example, the first sensor 210 may be stuck to the rim of a drum using a sticker. The sensors would then transmit data to a processor through a separate interface, which may be wired or wireless, where the data would then be interpreted.

Embodiments of the subject matter and the operations described in this specification can be implemented in digital electronic circuitry, or in computer software, firmware, or hardware, including the structures disclosed in this specification and their structural equivalents, or in combinations of one or more of them. Embodiments of the subject matter described in this specification can be implemented as one or more computer programs, i.e., one or more modules of computer program instructions, encoded on computer storage medium for execution by, or to control the operation of, data processing apparatus. Alternatively or in addition, the program instructions can be encoded on an artificially generated propagated signal, e.g., a machine-generated electrical, optical, or electromagnetic signal that is generated to encode information for transmission to suitable receiver apparatus for execution by a data processing apparatus. A computer storage medium can be, or be included in, a computer-readable storage device, a computer-readable storage substrate, a random or serial access memory array or device, or a combination of one or more of them. Moreover, while a computer storage medium is not a propagated signal, a computer storage medium can be a source or destination of computer program instructions encoded in an artificially generated propagated signal. The computer storage medium can also be, or be included in, one or more separate physical components or media (e.g., multiple CDs, disks, or other storage devices).

The operations described in this specification can be implemented as operations performed by a data processing apparatus on data stored on one or more computer-readable storage devices or received from other sources.

The term “data processing apparatus” and like terms encompass all kinds of apparatus, devices, and machines for processing data, including by way of example a programmable processor, a computer, a system on a chip, or multiple ones, or combinations, of the foregoing. The apparatus can include special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application specific integrated circuit). The apparatus can also include, in addition to hardware, code that creates an execution environment for the computer program in question, e.g., code that constitutes processor firmware, a protocol stack, a database management system, an operating system, a cross-platform runtime environment, a virtual machine, or a combination of one or more of them. The apparatus and execution environment can realize various different computing model infrastructures, such as web services, distributed computing and grid computing infrastructures.

A computer program (also known as a program, software, software application, script, or code) can be written in any form of programming language, including compiled or interpreted languages, declarative or procedural languages, and it can be deployed in any form, including as a standalone program or as a module, component, subroutine, object, or other unit suitable for use in a computing environment. A computer program may, but need not, correspond to a file in a file system. A program can be stored in a portion of a file that holds other programs or data (e.g., one or more scripts stored in a markup language document), in a single file dedicated to the program in question, or in multiple coordinated files (e.g., files that store one or more modules, sub programs, or portions of code). A computer program can be deployed to be executed on one computer or on multiple computers that are located at one site or distributed across multiple sites and interconnected by a communication network.

The processes and logic flows described in this specification can be performed by one or more programmable processors executing one or more computer programs to perform actions by operating on input data and generating output. The processes and logic flows can also be performed by, and apparatus can also be implemented as, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application specific integrated circuit).

Processors suitable for the execution of a computer program include, by way of example, both general and special purpose microprocessors, and any one or more processors of any kind of digital computer. Generally, a processor will receive instructions and data from a read only memory or a random access memory or both. The essential elements of a computer are a processor for performing actions in accordance with instructions and one or more memory devices for storing instructions and data. Generally, a computer will also include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., magnetic, magneto optical disks, or optical disks. However, a computer need not have such devices. Moreover, a computer can be embedded in another device, e.g., a mobile telephone, a personal digital assistant (PDA), a mobile audio or video player, a game console, a Global Positioning System (GPS) receiver, or a portable storage device (e.g., a universal serial bus (USB) flash drive), to name just a few. Devices suitable for storing computer program instructions and data include all forms of non volatile memory, media and memory devices, including by way of example semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks, e.g., internal hard disks or removable disks; magneto optical disks; and CD ROM and DVD-ROM disks. The processor and the memory can be supplemented by, or incorporated in, special purpose logic circuitry.

To provide for interaction with a user, embodiments of the subject matter described in this specification can be implemented on a computer having a display device, e.g., a CRT (cathode ray tube) or LCD (liquid crystal display) monitor, for displaying information to the user and a keyboard and a pointing device, e.g., a mouse or a trackball, by which the user can provide input to the computer. Other kinds of devices can be used to provide for interaction with a user as well; for example, feedback provided to the user can be any form of sensory feedback, e.g., visual feedback, auditory feedback, or tactile feedback; and input from the user can be received in any form, including acoustic, speech, or tactile input. In addition, a computer can interact with a user by sending documents to and receiving documents from a device that is used by the user; for example, by sending web pages to a web browser on a user's client device in response to requests received from the web browser.

Embodiments of the subject matter described in this specification can be implemented in a computing system that includes a back end component, e.g., as a data server, or that includes a middleware component, e.g., an application server, or that includes a front end component, e.g., a client computer having a graphical user interface or a Web browser through which a user can interact with an implementation of the subject matter described in this specification, or any combination of one or more such back end, middleware, or front end components. The components of the system can be interconnected by any form or medium of digital data communication, e.g., a communication network. Examples of communication networks include a local area network (“LAN”) and a wide area network (“WAN”), an inter-network (e.g., the Internet), and peer-to-peer networks (e.g., ad hoc peer-to-peer networks).

The computing system can include clients and servers. A client and server are generally remote from each other and typically interact through a communication network. The relationship of client and server arises by virtue of computer programs running on the respective computers and having a client-server relationship to each other. In some embodiments, a server transmits data (e.g., an HTML page) to a client device (e.g., for purposes of displaying data to and receiving user input from a user interacting with the client device). Data generated at the client device (e.g., a result of the user interaction) can be received from the client device at the server.

While this specification contains many specific implementation details, these should not be construed as limitations on the scope of any inventions or of what may be claimed, but rather as descriptions of features specific to particular embodiments of particular inventions. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the embodiments described above should not be understood as requiring such separation in all embodiments, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products.

While the present invention has been described at some length and with some particularity with respect to the several described embodiments, it is not intended that it should be limited to any such particulars or embodiments or any particular embodiment, but it is to be construed with references to the appended claims so as to provide the broadest possible interpretation of such claims in view of the prior art and, therefore, to effectively encompass the intended scope of the invention. Furthermore, the foregoing describes the invention in terms of embodiments foreseen by the inventor for which an enabling description was available, notwithstanding that insubstantial modifications of the invention, not presently foreseen, may nonetheless represent equivalents thereto. 

What is claimed is:
 1. A device for capturing vibrations produced by an object, the device comprising: a first sensor in contact with a surface of the object; and a second sensor spaced apart from the object and the first sensor and located relative to the object.
 2. The device of claim 1 further comprising a fixation element for fixing the device to the object, wherein the first sensor is placed in contact with the object by the fixation element and wherein the second sensor is located relative to the musical instrument by the fixation element.
 3. The device of claim 2 wherein the first sensor captures vibration in the object at the fixation element.
 4. The device of claim 3 wherein the first sensor is a piezoelectric sensor element.
 5. The device of claim 4 wherein the second sensor is a coil sensor.
 6. The device of claim 5 wherein the coil sensor is fixed relative to a magnet placed on a surface of the musical instrument.
 7. The device of claim 2 wherein the object is a drum, and wherein the first sensor contacts a rim of the drum and the second sensor is suspended over a head of the drum.
 8. The device of claim 7 wherein the fixation element is a clamp, and wherein the clamp has a first clamping surface for interfacing with a top surface of the rim of the drum and a second surface for interfacing with a bottom surface of the rim of the drum, and wherein the top surface contains the first sensor and the bottom surface has a first segment for gripping a first type of drum rim and a second segment for gripping a second type of drum rim.
 9. The device of claim 1 further comprising mixing circuitry for mixing a signal from the first sensor with a signal from the second sensor and an output for transmitting a mixed signal.
 10. A system for capturing vibrations produced by an object, the system comprising: a first sensor in contact with a surface of the object; a second sensor spaced apart from the first sensor and located relative to but not touching the object; a processor for identifying an audio event based on signals captured at the first sensor and the second sensor.
 11. The system of claim 10 further comprising an audio output for outputting a sound based on the audio event identified by the processor.
 12. A method for producing audio from electrical signals within a data processing device, the method comprising: receiving, at the data processing device, a stream of audio data; identifying, in the audio data, an onset of an audio event; selecting a discrete analysis window from the audio data based on the location of the onset of the audio event in the audio data; generating, by the data processing device, an n-dimensional representation of the audio event by evaluating the contents of the analysis window and recording the results of the analysis; classifying the audio event by comparing the n-dimensional representation of the audio event to expected representations of audio events along a plurality of the n dimensions; outputting a sound selected based on the classification of the audio event.
 13. The method of claim 12 wherein at least one dimension of the n-dimensional representation corresponds to a timbre characteristic of the audio event.
 14. The method of claim 12 wherein the audio data is a time domain audio signal and the evaluation of the contents of the analysis window is transformed to the frequency domain using a Constant-Q transform to separate the audio data into frequency bins.
 15. The method of claim 12 wherein the n-dimensional representation is compared geometrically to a plurality of audio zones defined by expected signal parameters in at least two of the n dimensions and associated with a sample audio event, and wherein when the n-dimensional representation is within one of the audio zones, the sound is a sample sound of a corresponding audio zone.
 16. The method of claim 15 wherein when the n-dimensional representation is not within one of the audio zones, the sound is a combination of elements of the sample sounds of a plurality of the audio zones based on a geometric distance between the n-dimensional representation and the audio zones along the at least two dimensions.
 17. The method of claim 16 wherein the n-dimensional representation is compared geometrically to a plurality of audio zones defined by expected signal parameters in at least two of the n dimensions, wherein each audio zone is associated with a sample sound, and wherein the sound is a combination of elements of the sample sounds of a plurality of the audio zones based on a geometric distance between the n-dimensional representation and the centers of the plurality of the audio zones along the at least two dimensions.
 18. The method of claim 17 wherein the audio zones correspond to sample signals corresponding to different playable surfaces of a drum or different modes of striking a drum.
 19. The method of claim 18 wherein a first of the audio zones corresponds to a sample recorded at a center of a drum head and a second of the audio zones corresponds to a sample recorded at a radial distance from the center of the drum head.
 20. The method of claim 18 wherein the sound is generated by blending the sample sounds from a plurality of audio zones based on a ratio of the geometric distances from each of the audio zones. 